Inorg. Chem. 1997, 36, 2341-2351
2341
Osmium Hydrazido and Dinitrogen Complexes George M. Coia, Martin Devenney, Peter S. White, and Thomas J. Meyer* Department of Chemistry, Venable and Kenan Laboratories, The University of North Carolina, Chapel Hill, North Carolina 27599-3290
David A. Wink Chemistry Section, Laboratory of Comparative Carcinogenesis, National Cancer Institute, Frederick Cancer Research and Development Center, Frederick, Maryland 21702 ReceiVed August 22, 1996X
[Os(tpy)(bpy)(NH3)](PF6)2 (1) was oxidized electrochemically in the presence of a series of amines in aqueous solutions buffered to pH 7. With secondary aliphatic amines, electrolysis gave [Os(tpy)(bpy)(NNR2)](PF6)3 (3); number of electrons n ) 4.6-5.0. 3 was reduced to [Os(tpy)(bpy)(NNR2)](PF6)2 (2) in aqueous and nonaqueous solutions with n ) 1.0. The structures of 2 were determined by X-ray crystallography for NR2 ) diethylamide (2a) and morpholide (2c) and were found to exhibit bent hydrazido(2-) coordination (Os-N-N ) 137°). The salts crystallized in the triclinic system, space group P1h. For 2a, a ) 9.004(1) Å, b ) 9.796(1) Å, c ) 20.710(2) Å, R ) 88.78(2)°, β ) 85.43(2)°, γ ) 86.22(2)°, and Z ) 2. For 2c, a ) 9.632(8) Å, b ) 21.229(9) Å, c ) 9.039(5) Å, R ) 97.41(4)°, β ) 94.28(5)°, γ ) 85.07(5)°, and Z ) 2. Solutions of 2 were protonated in strongly acidic media to give hydrazido(1-) complexes. The pKa of the protonated form of 2a is 0.90 ( 0.01. Reduction of 2 in aqueous solutions of pH 3.00σ(I) and 438 variable parameters. For 2c, data were collected on a Rigaku diffractometer by using the θ/2θ scan technique through a maximum 2θ ) 45°, giving 4782 collected reflections, all of which were unique. Cell dimensions were determined from 25 reflections in the range 20.03° < 2θ < 27.44° by least-squares analysis. The final least-squares cycle was calculated with 91 atoms, 418 parameters, and 3500 reflections of I > 2.50σ(I). Prior to refinement, diffraction data were corrected for absorption of Mo KR (linear absorption coefficient µ ) 36.9 cm-1). For 2a and 2c, corrections for Lorentz and polarization effects were applied, as well as a correction for secondary extinction (0.10 × 10-6). Resonance Raman spectra were obtained by use of an intensified charge coupled device (ICCD) detection system (Princeton Model 576G/ RB) with an ST 130 detector controller interfaced to an 80386-based computer. Data were collected and processed with Princeton Instruments CSMA software. Laser excitation at 441.6 nm was provided by a Liconix Model 4240NB HeCd laser while excitation at 488 nm was provided by a Spectra-Physics 165 Ar+ laser. Spectra were obtained from solutions which were millimolar in metal complex in NMR tubes with the laser beam focused onto the sample through a glass lens. The scattered light was collected with a 135° backscattering geometry by using a conventional camera lens and dispersion achieved with a SPEX 1877 triple spectrometer equipped with a 1200 groove/ mm grating. The ICCD was calibrated to the known Raman bands of solvents. Electrochemical measurements and preparative electrolyses were conducted by using a PAR Model 273 potentiostat. For voltammetry in aqueous solution, a glassy carbon disk working electrode and an SSCE reference electrode were used. For measurements in nonaqueous solution, the working electrode was a platinum disk, and the reference was a silver wire immersed in a CH3CN solution which was 0.01 M in AgNO3 and 0.1 M in tetra-n-butylammonium hexafluorophosphate (TBAH). For cyclic voltammetry experiments employing scan rates greater than 2 V/s, an analog waveform generator (PAR Model 175) was used to supply the triangle wave to the potentiostat. The analog current and voltage outputs of the potentiostat were sampled by a Tektronix Model 2230 digital storage oscilloscope (100 MHz). Acetonitrile was distilled from calcium hydride prior to use in electrochemical experiments. TBAH was recrystallized three times from absolute ethanol and dried in Vacuo. Buffers for aqueous voltammetry were prepared by neutralizing solutions of reagent A with solid or concentrated reagent B until the desired pH was reached. For pH 7.610.0, A ) 0.3 M NaOH, B ) H3BO3; for pH 5.8-7.8, A ) 0.5 M NaH2PO4, B ) NaOH; for pH 3.6-5.6, A ) 0.5 M CH3COOH, B ) NaOH; for pH 2.2-3.8, A ) 0.075 M potassium hydrogen phthalate, B ) H2SO4; for pH 1-2.2, A ) 0.5 M Na2SO4, B ) H2SO4. Electrolytes of pH 0.90. To reach desirable ratios the scan rate would have to be less than 25 mV/s, where convective effects become important.
Figure 3. Cyclic voltammograms of [Os(tpy)(bpy)(NNEt2)](PF6)2 (2a) recorded at pH 8.0 with a scan rate of 100 mV/s. The heavy line shows the effect of holding the potential for 30 s at the reductive limit prior to execution of the scan.
(NCCH3)]3+, which is paramagnetic with broad resonances over an extended chemical shift range, by cyclic voltammetry. The acetamide was the only product by NMR. Electrochemistry in Aqueous Solution. The oxidative electrochemistries of the osmium hydrazido and dinitrogen complexes were similar in aqueous solution and in acetonitrile. In each case, however, the Os(VI/V) wave was totally irreversible at ordinary scan rates. Oxidation (Epa ) +1.14 V for 4, cf. Table 3) of compounds 2-4 again afforded the OsIII solvento, in this case [Os(tpy)(bpy)(H2O)]3+, as confirmed by its pHdependent electrochemistry.9 Reduction of 2 occurs in two stages, as shown by the pair of irreversible, cathodic waves in the cyclic voltammetry. Both waves are present in aqueous solutions above pH 4, below which the reduction of protons begins to obscure the second wave. The products which emerge following reduction through each of these waves are different, as shown by the cyclic voltammograms in Figure 3 at pH 8.0. Reduction through wave II generates 1, as determined by comparison of Epa values with an authentic sample (Scheme 1). Reduction just past wave I generates none of the ammino complex, but instead gives an intermediate whose irreversible reoxidation occurs at Epa ) +0.14 V (wave A). By extending the anodic scan through wave D it is evident that the starting complex of 2 has been regenerated. We postulate that the intermediate produced at wave I and oxidized at wave A is the hydrazine complex [Os(tpy)(bpy)(NH2NR2)]2+ (eq 14) and that its oxidation regenerates the hydrazido. The second reductive feature (wave II) corre+2H+/2e-
[Os(tpy)(bpy)(NNR2)]2+ 98 [Os(tpy)(bpy)(NH2NR2)]2+ (14) sponds to reduction of the hydrazine complex by two electrons. (9) Pipes, D. W.; Meyer, T. J. Inorg. Chem. 1986, 25, 4042.
2346 Inorganic Chemistry, Vol. 36, No. 11, 1997 Scheme 1. Reactions of Hydrazido(2-) Complexesa
a t ) tpy; b ) bpy. The dashed line indicates that the process may be reversible only in the presence of high concentrations of the amine. Potentials above the line on the Latimer diagram are measured in aqueous solution, pH 7.0; potentials below the line are measured in CH3CN, 0.1 M in TBAH. All potentials are Vs SSCE.
+3H+/2e-
[Os(tpy)(bpy)(NH2NR2)]2+ 98 [Os(tpy)(bpy)(NH3)]2+ + H2NR2+ (15) Waves I, II, and A are observed through an extended pH range, but only in basic solutions (pH >7) is the cyclic voltammetry uncomplicated by the homogeneous decomposition of [Os(tpy)(bpy)(NH2NR2)]2+. In neutral and acidic solutions, reduction through wave II gives the ammino complex, but the distribution of products following reduction through wave I is a function of the pH and the scan rate. Voltammograms of 2a were recorded at several different scan rates in solutions of pH 5.0. (These appear in the Supporting Information.) Below pH 6, wave A is observed only at higher scan rates because decomposition of the hydrazine complex has become rapid on the cyclic voltammetry time scale. This process results in the appearance of 1 following reduction through wave I, and also in the enhancement of the peak current for wave I at the expense of wave II. Accordingly, the products of bulk reduction past the first reductive wave depend on the pH. In neutral and basic solutions, reduction gives 1 and an additional product, which is believed to be the hydrazine complex. However, at pH ) 0.5, the reduction occurs with n ) 4.0 ( 0.2. Under these conditions, 1 is obtained quantitatively. +5H+/4e-
[Os(tpy)(bpy)(NNR2)]2+ 98 [Os(tpy)(bpy)(NH3)]2+ + NH2R2+ (16) Hydrazine Complex. Decomposition of [Os(tpy)(bpy)(NH2NEt2)]2+, generated by electrochemical reduction of [Os(tpy)(bpy)(NNEt2)]2+ in basic solution, was studied spectrophotometrically over an extended pH range. Addition of acid (to pH ∼7) resulted in a color change, which was complete in 30 min. Precipitation of the products, followed by cation exchange chromatography, revealed that the osmium product was partitioned into equal amounts of 1 and 2a.
[Os(tpy)(bpy)(NH2NEt2)]2+ f [Os(tpy)(bpy)(NNR2)]2+ + [Os(tpy)(bpy)(NH3)]2+ + NH2R2+ (17) Throughout pH 1-10, the stoichiometry of eq 15 was preserved, as determined by UV-visible spectroscopy. The reaction is characterized by isosbestic points at 310, 344, 450, 520, 692, and 804 nm, which are not sensitive to the pH. In solutions buffered to pH 7.7 and below, the reaction was first-order in osmium. In this region, kobs was also first-order in [H+]. From
Coia et al. Table 4. Kinetic Data for the Decomposition of [Os(tpy)(bpy)(NH2NEt2)]2+ in Aqueous Solution pH
[Os], µM
[NHEt2], M
6.77 7.40 7.65 7.65 8.38 8.80 9.58 9.58
35.2 25.7 47.7 43.4 48.1 60.9 63.3 55.5
0 0 0 0.05 0 0 0 0.05
kobs, s-1 a 23.4 6.19 3.25 3.21 b 0.00480
ak b obs reported for runs obeying first-order kinetics. Fit to the Michaelis-Menten equation, -d[S]/dt ) k[S]/(KS + [S]), k ) 2.5 × 10-9 s-1, KS ) 1.5 × 10-5 M, with [S] representing the concentration of [Os(tpy)(bpy)(NH2NEt2)]2+.
the data in the first three entries of Table 4, a second-order rate constant of 220 M-1 s-1 was estimated. At higher pH, significant deviation from first-order kinetics occurred, with a gradual decrease in reaction order with respect to osmium as the solution became more basic. By pH 9.58, the reaction exhibited zero-order kinetics and was accurately fitted to the Michaelis-Menten equation (footnote b, Table 4). In the firstorder domain, neither the stoichiometry nor the rate was affected by diethylamine present in large excess. At higher pH, the rate was significantly supressed in the presence of diethylamine, where the observed kinetics were first-order with respect to osmium, but the stoichiometry remained unaffected. Hydrazido(1-) Complexes. It has been shown that hydrazido(2-) complexes, including those of the bent geometry, can be protonated or alkylated at NR to give hydrazido(1-) complexes.10,11 Accordingly, 2 reacts instantly with strong anhydrous acids in aprotic solvents to give bright yellow solutions.
[Os(tpy)(bpy)(NNR2)]2+ + H+ h [Os(tpy)(bpy)(NHNR2)]3+ (18) Cyclic voltammetry of a solution of 2a to which a drop of HBF4‚Et2O was added (Supporting Information) demonstrates the disappearance of the reversible oxidation wave. For the hydrazido(1-) species, the first oxidation, which is irreversible, occurs at a potential slightly higher than the Os(VI/V) wave of hydrazido(2-). Oxidation through this wave leads to the solvento complex. The effect on the electronic absorption spectrum is shown in Figure 4; the effect on the 1H-NMR specturm was mentioned earlier. The original spectra and voltammograms were recovered on addition of excess NaHCO3. Spectrophotometric titration of 2a in aqueous solution indicates that its pKa is 0.90 ( 0.01. Similar spectral changes were observed when small amounts of other potent electrophiles (triethyloxonium tetrafluoroborate, acetyl bromide) were added to solutions of 2. Regrettably, we have not been able to isolate any of the resulting hydrazido(1-) products in pure form. Vibrational Spectroscopy. The infrared absorption spectrum of 4 exhibits a very strong band at 2148 cm-1 both in KBr and in acetonitrile solution, which corresponds to νNN. By comparing infrared and resonance Raman spectra of the series of hydrazido complexes with those of [Os(tpy)(bpy)(L)](PF6)2 (L ) NH3, CH3CN) and [Os(tpy)(bpy)(NO)](PF6)3 as models, it was possible to identify bands which have their origins in the NNR2 ligands or which arise from coupling of NNR2 modes (10) Chatt, J.; Dilworth, J. R.; Dahlstrom, P.; Zubieta, J. A. J. Chem. Soc., Chem. Commun. 1986, 786. (11) Barrientos-Penna, C. F.; Einstein, F. W. B.; Jones, T.; Sutton, D. Inorg. Chem. 1982, 21, 2578.
Osmium Hydrazido and Dinitrogen Complexes
Inorganic Chemistry, Vol. 36, No. 11, 1997 2347 Table 5. Observed Band Positions (cm-1) from Resonance-Raman Spectra of Hydrazido(1-) Complexes in 6.0 M HCl or DCl (λex ) 441.6 nm) complex/solvent
ν1
ν2
ν3
2ν1
ν1 + ν 2
2a/HCl 2a/DCl 2b/HCl 2b/DCl 2c/HCl 2c/DCl
698 672 700 671 699 673
755 745 774 762 781 771
821 842b 842 842b 855 860b
1398 1344a 1397 1340 1398 1343
1452 1419 1475 1431 1477 1439a
a Incompletely resolved due to overlap with another band. Calculated value is given. b Weak relative to its intensity in the protonated complex.
Table 6. Observed and Calculated Isotopic Shifts (cm-1) for Resonance-Raman Bands in Table 5 complex
∆ν1(obs)a
∆ν1(calc)b
∆ν2(obs)a
∆ν3(obs)a
2a 2b 2c
-26 -29 -26
-21 -21 -21
-10 -12 -10
-4 0 +5
a
∆ν ) ν(DCl) - ν(HCl). b Calculated for a pure Os-NH stretch based on reduced mass of NH Vs ND. Table 7. Observed Band Positions (cm-1) from Infrared Spectra of Hydrazido(2-) Complexes in KBr
Figure 4. Comparison of the effects of various ligands on the electronic absorption spectra of OsII(tpy)(bpy) complexes. The dashed curve is a spectrum recorded in 6.0 M aqueous HCl. For all other spectra, the solvent was CH3CN. The vertical axis is in M-1 cm-1 and the horizontal axis λ in nm.
with pyridyl modes. Hydrazido(2-) salts 2 and 3 were studied by IR spectroscopy as suspensions in KBr. The protonated [hydrazido(1-)] forms were studied by resonance Raman in strongly acidic solutions. A resonance Raman spectrum of 2a was recorded in 6.0 M aqueous HCl (Supporting Information). Under these conditions, >99% of the complex exists in the protonated, hydrazido(1-) form. Upon excitation at 441.6 nm, the most intense features are a pair of bands (ν1, ν2) at 698 and 755 cm-1 and a weaker one at 821 cm-1. The same pattern of bands is observed for solutions of 2b and 2c in this medium, with ν2 and ν3 shifted in position, as shown in Table 5. Spectra of the same salts were recorded in 6.0 M DCl/D2O. The resulting shifts in band positions are listed in Table 6. On the basis of the large values of the isotopic shifts for ν1 and ν2, it is likely that these bands correspond to vibrational modes which involve significant displacement at NR. The frequency of ν2 is quite sensitive to the alkyl substituents at Nβ and shifts ∼10 cm-1 lower in the deuteriated complex. The ν3 band is also sensitive to the substituents at Nβ. While its frequency does not shift appreciably in the deuteriated solvent,
OsIV complex
νNN
OsV complex
νNN
2a 2b 2c
1259 1234 1219
3a 3b 3c
1332 1349 1340
its intensity decreases significantly. The ν1 band is rather insensitive to substituents at Nβ, but exhibits a large isotopic shift in each case. The shift is greater than calculated for an Os-NR stretch coupled to no other mode. The degree of resonance enhancement of ν1 and ν2 with excitation into the MLCT absorption at 441.6 nm is remarkable. The integrated intensity of ν1 in each complex is about 20 times that for the band at 1608 cm-1, a strong, polypyridyl-based band which appears in all OsII(tpy)(bpy) complexes. This suggests that, for these modes, there is a considerable change in equilibrium displacement between ground and excited states. We are not able to assign these bands definitively, but on the basis of their large isotopic shifts and strong resonance enhancements, they are likely to involve Os-NR stretching, or in-plane or out-of-plane bending of the Os-NR-Nβ unit. Coupling to other local coordinates gives rise to ν1, which exhibits minimal displacement of atoms except the NRH group, and ν2, which involves displacement of the entire NHNR2 group. Strong overtone and combination bands are observed at 2ν1 and ν1 + ν2. These are the most intense features in the 12001500 cm-1 region, where N-N stretching bands for hydrazido ligands in other complexes have been reported. All other bands observed in this region have their origins in the polypyridyl ligands. A pair of weak bands between 1215 and 1275 cm-1 show marked intensity variations depending on whether NR is protonated or deuteriated. This points to some degree of coupling of the pyridyl-based modes with NR-H. It is significant that no band in the spectra could be identified as an N-N stretch. The infrared spectrum of 2 suspended in KBr does show a strong band which is not present in the model complexes and which is sensitive to the substituents at Nβ. Its position makes it a likely candidate for νNN. This band disappears in the oxidized form (3); it reappears ca. 100 cm-1 higher in frequency and again shows a dependence on the nature of R, Table 7.
2348 Inorganic Chemistry, Vol. 36, No. 11, 1997
Coia et al.
Values of νNN in the range 1315-1510 cm-1 have been obtained in the IR spectra of other hydrazido(2-) complexes,12 although a comparison is not completely apt since there are no stretching frequencies reported for bent hydrazido complexes.
the putative OsII hydrazine intermediate persists. Recently, η1coordination in an OsII hydrazine complex has been established by X-ray crystallography.19 This complex also undergoes a two-proton, two-electron oxidation,
Discussion
[Os(CO)2(PPh3)2(Br)(NH2NH2)]+ 98
Pb(OAc)4
Investigations of the reactivity of very electrophilic imido (or nitrene) complexes began with Basolo’s experiments on the decomposition of transition metal azides.13
[M(NH3)5(N3)]2+ + H+ f [M(NH3)5(NH)]3+ + N2 (19) M ) Ru, Ir Since that time, there have been few well-characterized examples of electrophilic NH2- complexes,14 in contrast to the myriad of examples (found mainly in the chemistries of the earlier, more electron rich transition metals) in which the nitrogen is nucleophilic.15 It appears that eq 4 is the first example of the formation of a hydrazine complex by nucleophilic attack of a secondary amine on an NH group. At the anodic potentials where the OsIV imido is generated, the OsII hydrazine complex is further oxidized to the OsV hydrazido(2-). Although the imido reacts under the same conditions with primary amines and with ammonia also, these reactions are not suitable for generating mono- or unsubstituted hydrazido complexes, as these species, once formed, are further oxidized to the OsII dinitrogen complex. Transition metal hydrazido complexes maintain central importance as models for enzymatic nitrogen fixation. Most of this work has been confined to the electron-rich coordination spheres of molybdenum and tungsten, which readily accommodate the coordination of dinitrogen.16 Dinitrogen complexes have been protonated to afford hydrazido(2-) complexes, which have in turn been reduced under N2 to yield ammonia and the parent dinitrogen complex. Although the goal of an electrocatalytic cycle has not yet been fully realized, systems have been developed which survive through several cycles of fixation and reduction.17 Recent efforts have focused on more electron rich environments, in which the expected intermediates, hydrazine and hydrazido(1-) complexes, can be isolated and their decomposition followed in a stepwise manner.18
W(Cp*)Me3(η2-NH2NH2)+ 98 +e-
W(Cp*)Me3(η2-NH2NH2) f W(Cp*)Me3(NH) + NH3 (20) The present work shows that the reduction of polypyridyl OsIV dialkylhydrazido(2-) complexes is also a stepwise process. It proceeds through a two-electron intermediate and results in fission of the N-N bond. The first stage of reduction (eq 12) is chemically reversible in neutral and basic solutions, where (12) Chatt, J.; Diamantis, A. A.; Heath, G. A.; Hooper, N. E.; Leigh, G. J. J. Chem. Soc., Dalton Trans. 1977, 688. (13) (a) Kane-Maguire, L. A. P.; Sheridan, P. S.; Basolo, F.; Pearson, R. G. J. Am. Chem. Soc. 1970, 92, 5865. (b) Lane, B. C.; McDonald, J. W.; Basolo, F.; Pearson, R. G. J. Am. Chem. Soc. 1972, 94, 3786. (c) Gafney, H. D.; Reed, J. L.; Basolo, F. J. Am. Chem. Soc. 1973, 95, 7998. (14) Perez, P. J.; Luan, L; White, P. S.; Brookhart, M.; Templeton, J. L. J. Am. Chem. Soc. 1992, 114, 7929 and references 1-5 within. (15) (a) Nugent, W. A.; Haymore, B. L. Coord. Chem. ReV. 1980, 31, 123. (b) Glueck, D. S.; Hollander, F. J.; Bergman, R. G. J. Am. Chem. Soc. 1989, 111, 2719, and references 1 and 2 within (17 articles).
[Os(CO)2(PPh3)2(Br)(NHdNH)]+ (21) but the product is an η1-trans-diazene complex. In the cyclic voltammetry of [Os(tpy)(bpy)(NH2NEt2)]2+ (Figure 3), the oxidative wave (A) is separated from the reductive wave (I) by nearly 900 mV. The high degree of electrochemical irreversibility of this couple can be attributed to the demands of proton transfer on the electron-transfer kinetics. A similar effect has been observed for the chemically reversible oxidation of OsIV(tpy)(Cl)2(NH3) to [OsVI(tpy)(Cl)2(N)]+, for which oxidative and reductive waves are separated by as much as 1400 mV.20 The thermodynamic potential of the hydrazido/hydrazine couple, +2H+/2e-
[OsIV(tpy)(bpy)(NNR2)]2+ y\ z + + -2H /2e
[OsII(tpy)(bpy)(NH2NR2)]2+ (22) estimated from the peak potentials of waves I and A, is -0.20 V at pH 7.0. It exhibits a proton dependence of -62 mV/pH unit over the range 5 < pH < 9, consistent with a 2H+/2ecouple. Wave I of Figure 3 may actually be a superposition of two one-electron waves. If so, the reduction is reminiscent of that of polypyridylruthenium oxo complexes such as [Ru(bpy)2(py)(O)]2+, which exhibits two closely spaced waves. We hesitate to make this claim for the present system as the resolution of the two waves is dependent on the composition of the electrolyte and could be due to adsorptive effects. In the second stage of reduction (eq 13), coordinated hydrazine is reduced to ammonia, and the secondary amine is released. This results in the net reversal of the coupling process by which the hydrazido complexes were formed. The appearance of wave E in Figure 3 suggests that capture of the H2NR2+ cation is efficient within the confines of the diffusion layer even when the amine is not present in excess. A complicating factor in the reductive electrochemistry of 2 is the instability of the OsII hydrazine intermediate. It gives rise to an additional pathway for N-N bond cleavage, apart from direct reduction of the hydrazine complex at the electrode. Below pH 6, decomposition is quite rapid, and [Os(tpy)(bpy)(NH3)]2+ is detected in the cyclic voltammetry of 2 after cycling through only the first reductive wave. Our initial observation of kinetics first order in both [Os] and [H+] led us to suppose that the first step was protonation, followed by rate-determining decomposition of the protonated intermediate, i.e., (16) (a) McCleverty, J. A. Transition Met. Chem. 1987, 12, 282. (b) Johnson, B. F. G.; Haymore, B. L.; Dilworth, J. R. In ComprehensiVe Coordination Chemistry; Wilkinson, G., Gillard, R. D., McCleverty, J. A., Eds.; Pergamon Press: Oxford, 1987; Vol. 2, Section 13.3.7. (c) Henderson, R. A.; Leigh, G. J.; Pickett, C. J. AdV. Inorg. Chem. Radiochem. 1983, 27, 197. (17) Pickett, C. J.; Ryder, K. S.; Talarmin, J. J. Chem. Soc., Dalton Trans. 1986, 1453. (18) Schrock, R. R.; Glassman, T. E.; Vale, M. G. J. Am. Chem. Soc. 1991, 113, 725. (19) Cheng, T.-Y.; Ponce, A.; Rheingold, A. L; Hillhouse, G. L. Angew. Chem., Int. Ed. Engl. 1994, 33, 657. (20) Pipes, D. W.; Bakir, M.; Vitols, S. E.; Hodgson, D. J.; Meyer, T. J. J. Am. Chem. Soc. 1990, 112, 5507.
Osmium Hydrazido and Dinitrogen Complexes
Inorganic Chemistry, Vol. 36, No. 11, 1997 2349
[Os(tpy)(bpy)(NH2NR2)]2+ + H+ h [Os(tpy)(bpy)(NH2NR2H)]3+ (23)
[Os(tpy)(bpy)(NNR2)]4+ + S f
[Os(tpy)(bpy)(NH2NR2H)]3+ h [Os(tpy)(bpy)(NH)]2+ + NH2R2+ (24) 2+
[Os(tpy)(bpy)(NH)]
2+ fast
+ [Os(tpy)(bpy)(NH2NR2)] 2+
[Os(tpy)(bpy)(NH3)]
A second possibility is that [Os(tpy)(bpy)(NNR2)]4+ may spontaneously solvolyze by loss of the hydrazido ligand,
98 2+
+ [Os(tpy)(bpy)(NNR2)]
(25)
Precedence for eq 24 is found in the decomposition of MCp*Me3(η2-NH2NH2)+ (M ) Mo, W; Cp* ) C5Me5), which yields the imido.21 The reverse reaction in eq 23 is known to be efficient, since it is part of the sequence of reactions by which the hydrazido complexes are formed. However, it was found that throughout the first-order domain, addition of large amounts of NH2Et2+ to the reaction mixture did not retard the rate. Mechanistic investigations of this system are still ongoing. We have not yet arrived at a generalized rate law which describes the decomposition kinetics over an extended pH range. At present, it is valid to conclude only that there are two parallel pathways by which eq 16 is achieved. One involves ratedetermining, first-order decomposition of an intermediate which is protonated once with respect to the starting complex. The other pathway is mediated by an unknown catalyst and gives rise to slow, zero-order kinetics when the concentration of protons is low. While added diethylamine does not influence the rate in the first-order domain, at higher pH it suppresses the catalytic branch of the reaction. At pH 9.58, with 0.05 M added diethylamine, kobs is in good agreement with the value extrapolated from lower pH. Oxidation of 2 occurs Via two reversible, one-electron steps. The product of the first oxidation (3) is a stable compound which can be isolated in solid form, while the product of the second oxidation was observed only on the rapid time scales of cyclic voltammetry. Both in aqueous solution and in acetonitrile, its decomposition affords the OsIII solvento complex. When this species, generated by oxidation of 2a, is allowed to decompose in CD3CN solution, the organic which forms is the product of carbocation attack on the solvent.22
[Os(tpy)(bpy)(S)]3+ + NNR2+ (31) with decomposition of NNR2+ at the anode giving rise to carbocation-addition products. In the bulk electrolysis experiment, oxidation of 3 consumes ca. 2 equiv of electrons, which agrees with the net reaction,
[Os(tpy)(bpy)(NNR2)]3+ + CH3CN 98 -e-
[Os(tpy)(bpy)(NCCH3)]3+ + N2 + 2R+ (32)
(30)
By cyclic voltammetry, the peak height of the second oxidation wave is equal to that of the first, suggesting that on a timescale of seconds, only one electron is involved in the oxidation of 3. For the former mechanism, this implies that the alkyldiazonium intermediate of eq 27 is relatively long-lived, which is not consistent with the prompt appearance of the [Os(tpy)(bpy)(NCCH3)]3+/2+ product wave on the return scan. For the latter mechanism, the implication is only that the liberated NNR2+ fragment (or more likely an intermediate decomposition product) persists for a period of seconds prior to its annihilation at the electrode. We have made no effort to confirm this; however, the decomposition of [Os(tpy)(bpy)(NNR2)]4+ by ligand loss (eq 31) is fully consistent with the electrochemical results and is readily explained on the basis of electronic arguments which are developed later in this section. Since 2a and 2c are the first terminal hydrazides reported for osmium, some discussion of the structures is warranted. Apart from the unusual bent geometry of the NNR2 ligand, both structures are unexceptional. The bipyridine in 2a and 2c is bound somewhat asymmetrically, with the Os-N(bpy) bond trans to the hydrazido ∼0.1 Å shorter than the bond cis. Multiple bonding between osmium and the hydrazido ligand is revealed by short Os-NR distances in 2a and 2c. This distance (1.89 Å for 2a, 1.84 Å for 2c) is longer than the 1.78-1.82 Å commonly observed for Mo-N and W-N in linear NNR2 complexes. It is a feature shared by the other three structurally characterized compounds containing the bent hydrazido ligand (M-N ) 1.84-1.92 Å).23 The Os-N-N bond angle of 137° (for 2a and 2c) is also within the 131-146° range established by these complexes. As first noted by Sutton et al., the M-N-N geometry in hydrazido(2-) complexes is such as to allow the maximum number of donated electron pairs consistent with achieving a valence electron count of 18.11 In the more common linear arrangement, NNR22- is a six-electron donor, whereas the bent configuration requires four bonding electrons and one lone pair on NR. The Os-N-N bond angle in 2a and 2c indicates that two pairs of electrons (σ + π) are donated to the metal. Moreover, the NNCC units of the hydrazido ligands are planar, suggesting a π interaction between nitrogens. The variation in the N-N bond lengths of the osmium complexes is striking. Complex 2c has the longest N-N bond reported for a hydrazido-
decomposition proceeds as in eq 9. Potentials sufficient to access the Os(VI/V) wave are also high enough to oxidize [OsII(tpy)(bpy)(N2)]2+ to OsIII, where dinitrogen is rapidly replaced by solvent (eqs 29 and 30).
(21) Schrock, R. R.; Glassman, T. E.; Vale, M. G. J. Am. Chem. Soc. 1991, 113, 725. (22) The Ritter reaction. See: March, J. AdVanced Organic Chemistry, 3rd ed.; Wiley: New York, 1985; Chapter 6, p 860. (23) (a) Jones, T.; Hanlan, A. J. L.; Einstein, F. W. B.; Sutton, D. J. Chem. Soc., Chem. Commun. 1980, 1978. (b) Reference 11. (c) Dilworth, J. R.; Harrison, S. A.; Walton, D. R. M.; Schweda, E. Inorg. Chem. 1985, 24, 2595.
We have considered two possible mechanisms to account for these observations. The first begins with carbocation loss from OsVI (eq 27). Once the first alkyl group is lost, subsequent
[Os(tpy)(bpy)(NNR2)]4+ f [Os(tpy)(bpy)(NNR)]3+ + R+ (27) [Os(tpy)(bpy)(NNR)]3+ f [Os(tpy)(bpy)(N2)]2+ + R+ (28) [Os(tpy)(bpy)(N2)]2+ 98 [Os(tpy)(bpy)(N2)]3+ (29) -e-
[Os(tpy)(bpy)(N2)]3+ f [Os(tpy)(bpy)(S)]3+ + N2 (S ) CH3CN, H2O)
2350 Inorganic Chemistry, Vol. 36, No. 11, 1997 (2-) complex, while 2a has the second shortest.24 This result is surprising, given the similarity of the complexes in other ways. Consistently, however, the N-N stretching frequency in 2c is 50 cm-1 lower than in 2a. The barrier to rotation about the N-N bond in 2a is 55.9 kJ mol-1 at room temperature. Symetrically substituted N,N-dialkylhydrazido(2-) ligands are rare, so there is little precedent with which to compare this result. Instances of the bent variety were unknown prior to this study. Among linear dimethylhydrazido(2-) complexes which contain the molybdenum oxo core, methyl groups can be inequivalent, as in [Mo(O)(SPh3)(NNMe2)]-,25 or equivalent, as in Mo(O)(dmdtc)2(NNMe2).26 In the complex Mo{HB(Me2pz)3(NO)(I)(NMeNMe2)}, which contains a hydrazido(1-) ligand, the methyl resonances of Nβ are equivalent at room temperature but separate at low temperature. The barrier to rotation was estimated to be 39 kJ mol-1. The hydrazido(1-) complex derived from 2a exhibits a much larger barrier. Its ethyl groups remain inequivalent up to the decomposition temperature.27 We refer to compounds 2 as hydrazido(2-) complexes of OsIV because the dianionic ligand can be formally derived from N,N-dialkylhydrazine by removal of two protons. This formalism places three unshared electron pairs on the R nitrogen. The bonding picture is completed by allowing one of these to interact with that of the β nitrogen, resulting in filled π and π* combinations. The π* orbital so formed donates into the vacant dπ orbital of OsIV. The π interaction gives rise to the short N-N bond length (1.25 Å for 2a, 1.40 Å for 2c) and planarity of the NNCC fragment. However, it is equally valid to envision the bonding as resulting from the interaction of a π-accepting NNR2 (isodiazene) molecule with OsII. Our preference is for the latter description, on the basis of the properties of these molecules. The electronic absorption spectra of OsII complexes containing bipyridine and terpyridine ligands exhibit strong ( ∼ 105) metal-to-ligand charge-transfer (MLCT) bands in the visible. This feature is typically not shared by osmium complexes in oxidation state III or higher, whose visible absorptions typically arise from ligand-to-metal charge-transfer (LMCT) or d-d transitions, with MLCT bands appearing at higher energy.28 In the OsII complexes, stabilization of the metal dπ orbitals by interaction with π-acid ligands brings about an increase in the MLCT energy. The effect can be substantial, as demonstrated by the absorption spectrum of [Os(tpy)(bpy)(NO)](PF6)3 in Figure 4. The powerfully π accepting nitrosyl ligand shifts all of the MLCT bands into the near UV. The complexes [Os(tpy)(bpy)(NNR2)]2+ have intense absorption bands in the visible, and these shift to higher energy when the hydrazido ligand is protonated at NR. If these bands correspond to MLCT transitions, then the observed shift is consistent with the anticipated increase in π-accepting ability of NHNR2+ over NNR2. This effect is also apparent in the electrochemistry of the complexes. On protonation, the Os(III/II) wave of 2a shifts anodically by 1 V and becomes irreversible. This is a direct indication of the instability of the (24) The shortest N-N bond length is reported in the following: Dilworth, J. R.; Zubieta, J. A.; Hyde, P. R. J. Am. Chem. Soc. 1982, 104, 365. (25) Burt, R. J.; Dilworth, J. R.; Leigh, G. J.; Zubieta, J. A. J. Chem. Soc., Dalton Trans. 1982, 2295. (26) Bishop, M. W.; Chatt, J.; Dilworth, J. R.; Hursthouse, M. B.; Montevalli, M. J. Chem. Soc., Dalton Trans. 1979, 1600. (27) There is another case in which structural isomers were observed for a rhenium hydrazido(1-) but not for the bent hydrazido(2-) from which it was derived (Inorg. Chem. 1984, 23, 363). In this case, the alkyl substituents of Nβ are inequivalent, and it is not clear whether protonation increases or decreases the barrier. (28) (a) Kober, E. M.; Meyer, T. J. Inorg. Chem. 1982, 21, 3967. (b) Kober, E. M. Ph.D. Dissertation, University of North Carolina at Chapel Hill, 1982.
Coia et al.
Figure 5. Energy level diagram for the cation in 2a, based on extended Hu¨ckel calculations. The z axis is the one which passes through Os and NR; the yz plane contains Os, NR, and Nβ.
OsIII state in the hydrazido(1-) species due to the electronwithdrawing nature of the protonated ligand. The electronic structure of these systems was further investigated by means of extended Hu¨ckel (EH) calculations. The HOMO and LUMO of the diethylisodiazene molecule are represented in Figure 5. The HOMO, in terms of the valence bond model, is a major contributor to the lone electron pairs of NR and to the C-N σ bonds. The LUMO is purely π*(NN). When this species becomes a ligand in 2a, all of its occupied orbitals lie below the dπ levels of osmium, and π*(NN) lies above them. Thus, the formal oxidation state of the metal is +2. The LUMO of the complex cation is largely the same as in the free NNEt2 molecule, but it contains a significant contribution from dxz. Accordingly, dxz is not a pure metalbased orbital, but exhibits substantial delocalization onto the isodiazene ligand. The splitting of the dπ orbitals is a function of their interaction with the π* orbitals of each of the ligands. The symmetry of dxz allows it to interact with π* levels from three pyridyls and the isodiazene ligand. This orbital lies lowest in energy, immediately below dxy and dyz, which is the HOMO. The presence of two filled dπ levels which do not mix with the orbitals of the isodiazene ligand suggests that both stages of oxidation observed for [Os(tpy)(bpy)(NNR2)]2+ are primarily metal based, Viz., Os(III/II) and Os(IV/III). Loss of the back-bonding isodiazene ligand is observed at the OsIV stage, when the dπ orbitals become too contracted to support π bonding. In comparison, oxidation to OsIII is sufficient to cause lability of the π-acid ligands in [Os(tpy)(bpy)(N2)]3+ and [Os(tpy)(bpy)(NHNR2)]4+. Also borne out in the MO diagram of Figure 5 is the stepwise nature of the reductive chemistry. Reduction by two electrons fills π*(NN), reducing the N-N bond order to unity. The parent pz(N) orbitals are no longer constrained to interact, and they degenerate into the two lone electron pairs of hydrazine. Further reduction results in cleavage of the N-N bond. Because the HOMO is a dπ orbital that does not mix with π*(NN), the EH treatment does not predict a change in N-N bond order on oxidation of 2 to 3. Experimentally, νNN is ca. 100 cm-1 higher in 3 than in 2, which may be due to decreased dπ + π*(NN) overlap in the oxidized complex. Since the dependence of orbital extension (ζ) on the atomic charge is not represented in the EH model, we cannot presently elucidate this effect. The same issue arises with the protonation of 2. The barrier to rotation about N-N is greater in the hydrazido(1-) species than in the hydrazido(2-), suggesting a possible increase
Osmium Hydrazido and Dinitrogen Complexes in the N-N bond order. Although the lone pair which is the site of protonation is not directly tied into the π system of the ligand, protonation affects the amount of positive charge residing on the metal. Overlap between dπ and π*(NN) may play a role here as well. While the origin of the 700-800 cm-1 bands in the Raman spectra of these species has not been uniquely determined, it is clear from their resonance enhancements that these modes are strongly coupled to the transition between ground and excited states. If the bands correspond to the Os-NR stretch coupled to other modes, and if the electronic transition is MLCT in character, a large resonance enhancement is indeed expected on the basis of the influence of the oxidation state of the metal on equilibrium bond lengths. An Os-N stretching frequency in the 700-800 cm-1 range is suggestive of substantial metalligand multiple bonding which, according to the isodiazene description, must be a result of the back-bonding interaction.
Inorganic Chemistry, Vol. 36, No. 11, 1997 2351 Acknowledgment. The authors are deeply appreciative of Dr. John C. Brewer and Dr. Durwin Striplin for their helpful discussions and Dr. William M. Davis (MIT) for solution and refinement of the structure of 2a. We gratefully acknowledge the National Institutes of Health for supporting this research through Grant No. 5-RO1-GM-32296-05. Supporting Information Available: Tables of collection and refinement parameters, positional and thermal parameters, bond distances, and bond angles for 2a and 2c, background-subtracted cyclic voltammograms of 2a recorded at pH 5.0, cyclic voltammograms of 2a recorded in CH3CN with 0.1 M TBAH at a scan rate of 100 mV/s and resonance-Raman spectra of 2a in 6.0 M HCl and 6.0 M DCl solutions (35 pages). Ordering information is given on any current masthead page. IC961025V
